What Is Air Entrainment in Concrete: Enhancing Construction

What is air entrainment in concrete? It’s the deliberate integration of microscopic air bubbles to improve the mix’s resistance to crack under freeze-thaw cycles. These crucial bubbles are introduced through air-entraining agents, enhancing the concrete’s longevity especially in colder climates. This article will guide you through the why and how of air entrainment, making its importance in durable construction clear and understandable.

Key Takeaways

Learning what is air entrainment in concrete involves discovering tiny uniform air bubbles to enhance durability and resistance to freeze-thaw damage, especially in cold climates.
The air-entrained concrete is created by mixing air-entraining admixtures into the mix, which slightly reduces tensile and compressive strength, but significantly improves freeze-thaw resistance and workability.
The air content in concrete must be rigorously tested and adjusted on-site to ensure optimal performance and durability, considering factors such as mix composition, temperature, and aggregate size.

Understanding What is Air Entrainment in Concrete

Despite how it sounds, asking and discovering what is air entrainment in concrete is quite straightforward. Initially discovered by accident, air-entrained concrete was observed to contain tiny, dispersed air bubbles that enhanced its durability and resistance to freezing and thawing. Unlike non-air-entrained concrete, which has larger and irregularly shaped air pockets, air-entrained concrete includes a deliberate addition of an air-entraining agent that significantly improves its durability and freeze-thaw resistance.

Over time, the practice of air entrainment, which involves entraining air, has become a standard in the construction industry, particularly in regions with cold climates, due to its observed durability benefits.

Definition and Purpose: What is Air Entrainment in Concrete

Air-entrained concrete is characterized by the presence of microscopic air bubbles, distributed uniformly throughout the material. These tiny air bubbles serve a noble purpose. They act as internal cushioning in the concrete, absorbing energy during impacts and increasing resistance of ordinary concrete to physical forces, which aid in preventing cracks and breaking.

Hence, deliberately creating air bubbles in concrete bolsters its durability and workability, fortifying it against external factors.

Impact on Concrete Properties: What is Air Entrainment in Concrete

When air-entraining admixtures are added to a concrete mix, they create millions of non-coalescing air bubbles, enhancing the material’s resistance to moisture due to freeze-thaw cycles. This process does slightly decrease both tensile and compressive strength, but the reduction in tensile strength is generally less pronounced.

Interestingly, air entrainment can positively affect the flexural strength in lean concrete mixes, allowing for water content reduction and the use of smaller aggregate sizes. Ultimately, air entrainment creates a more cohesive concrete mix with reduced water segregation, partially offsetting the loss in compressive strength due excess water due to increased workability.

The Process of Air Entrainment

Air entrainment in concrete is no mere happenstance; it’s a deliberate process achieved by using air-entraining admixtures. These admixtures create microscopic air bubbles that provide small pockets for water to expand into during freeze-thaw conditions. The typical dosage of these admixtures is usually about 0.2–0.4% based on cement weight.

The outcome is a concrete mix permeated with minuscule, meticulously entrained air bubbles, bolstering the material’s durability and freeze-thaw resilience in freezing temperatures.

Air-Entraining Admixtures

Air-entraining admixtures are substances used to create stable air bubbles in concrete. They range from natural wood resins and alkali salts to animal and vegetable fats and oils, such as Vinsol resin, Darex, Teepol, and Chemical. The designers tune these admixtures for specific mix designs. For instance, Vinsol resins derived from wood suit low water-cement mix ratios, which are important in road pavements. In contrast, synthetic resins can produce smaller bubbles but may cause excessive air entrainment in high slump conditions.

Emerging admixtures like MasterSphere, based on specially developed superabsorbent polymers, offer a promise for more reliable air-entrained concrete with enhanced freeze-thaw durability.

Mixing Process

The introduction of air-entraining admixtures into the concrete mix merely marks the initial phase. Once added, the mix must be stirred for at least 30 revolutions at mixing speed to ensure even dispersion of air bubbles throughout the mix. This mixing process is crucial for optimal air entrainment.

Interestingly, the application of vibration mixing can shorten the time needed to achieve a uniform concrete mixture and even enhance the entrainment of air.

Key Benefits of Air-Entrained Concrete

Air-entrained concrete brings a host of benefits to the table, making it an ideal choice for various construction scenarios. Its main advantages include improved freeze-thaw resistance, workability, and durability. The durability of concrete is significantly improved by air entrainment, especially for structures exposed to frequent freeze-thaw cycles. Air-entrained concrete also resists surface scaling caused by de-icing chemicals and salts.

Moreover, learning about what is air entrainment in concrete offers several benefits:

It enhances workability, making the concrete paste easier to handle and place.
The porosity introduced by air-entrained concrete reduces water and liquid permeability, preventing cracking and fissures.
All these benefits optimize its use in diverse construction scenarios.

Freeze-Thaw Resistance

One of the primary benefits of air-entrained concrete is its improved resistance to freeze-thaw cycles. When water freezes, it expands. In non-air-entrained concrete, this expansion can cause internal distress, leading to cracks and scaling. However, the microscopic air cells in air-entrained concrete provide space for this water expansion, thus reducing damage from internal pressure.

Air-entrained concrete significantly outlasts non-air-entrained concrete through numerous freeze-thaw cycles, enhancing the lifespan of structures in cold climates. It also exhibits resistance to scaling in harsh weather conditions and chemical deicers due to a more robust microstructure.

Improved Workability

Workability refers to how easy it is to handle, place, and finish concrete. Air entrainment facilitates enhanced workability of the concrete without necessarily increasing the water-cement ratio. The plasticity of air-entrained concrete mixtures is increased, allowing for easier sliding with less friction, which aids in handling such as pumping and transporting.

This improved fluidity also reduces the risks of segregation, bleeding, and cracking in the concrete.

Increased Durability

Air-entrained concrete isn’t just easier to work with; it’s also more durable. The integration of air-entraining agents or air-entraining Portland cement introduces stabilized voids within the concrete, enhancing its durability after hardening. It exhibits increased resistance to sulfate attacks, and alkali-silica reactivity, and suffers less from freeze/thaw cycles, which helps to maintain structural integrity over time.

Innovations like MasterSphere contribute to sustainable construction by offering freeze-thaw resistance even with clinker-efficient types of cement and by leaving air-filled voids in the hardened concrete.

Factors Influencing Air Entrainment

Discovering what is air entrainment in concrete isn’t a one-size-fits-all process. Several factors can impact the process and the resulting properties of the concrete. These include:

Mixing time
Composition
Consistency
The size of aggregates
Temperature

Understanding these influencing factors is vital for achieving the desired quality and performance of the concrete.

Composition and Consistency

The composition and consistency of a concrete mix have a pivotal role in air entrainment. This is influenced by:

The type and properties of cement
Water-cement ratio
Types of aggregates
The presence and dosage of admixtures

Air entrainment can lower the water content needed for a specific slump value, which can, in turn, enhance the concrete’s strength and durability without compromising workability.

However, while incorporating air-entraining admixtures, a balance must be struck between achieving the desired workability and preserving the strength.

Aggregate Size

The size of aggregates used in a concrete mix can also impact air bubble entrainment. Concrete mixes with different aggregate sizes require varying percentages of air entrainment to achieve uniform bubble size and spacing. The size distribution of aggregates influences the stability of entrained air bubbles; larger aggregates could lead to a more unstable air void system.

An excessive amount of large-sized aggregates or disproportionate use of reinforcement in a significant portion of the mix can impede concrete flow and create voids.

Testing and Adjusting Air Content in Concrete

For optimal quality and performance, precise testing and adjustment of air content in concrete are necessary. It is essential to test the air content of fresh concrete at the job site because an entrained air content higher or lower than specified can adversely impact concrete performance. This ensures that the concrete meets the required standards for quality and durability..

The Type B pressure meter is commonly utilized for measuring the air content of normal-weight concrete. This device is designed specifically for this purpose.

Testing Methods

Various methods are used to test air content in concrete, each with its pros and cons. The pressure meter method uses Type A or Type B concrete air meters to measure volume differences in a concrete specimen before and after compression. However, pressure tests can result in errors, often indicating higher air content than present.

The volumetric method involves washing the air voids out of a known volume of concrete in a container via agitation with a sealed cover and graduated markings filled with fluid. In the gravimetric testing method, a concrete sample’s weight is used to calculate its unit weight and determine air content. Regular calibration and proper use of testing meters are crucial for obtaining precise results in air content measurements.

Adjusting Air Content

Often, adjustments are necessary to attain the desired level of entrained air in concrete. If preliminary testing of concrete at the job site shows air content below the specified level, add additional air-entraining admixture to achieve the desired air-content level. Adjust inconsistent air content in concrete by modifying the amount of air-entraining admixtures or altering the mixing process to ensure the desired level of entrained air.

However, if air content is too high upon arrival at the job site, especially for wet loads, measure it before placing the concrete to confirm it meets the specified requirements and to avoid issues like difficulty finishing or susceptibility to scaling.

Challenges and Solutions in Air Entrainment

Though air entrainment provides manifold benefits, it also presents its share of challenges. Issues such as honeycombing and fly ash interference can complicate the process of air entrainment in concrete. Improper air entrainment can cause significant issues like honeycombing, where entrapped air creates larger, unevenly distributed voids, impacting structural integrity.

Fortunately, proper placement techniques and mixture designs can effectively address these challenges, ensuring even air distribution and enhancing construction durability through air entrainment.

Entrapped Air and Honeycombing

Honeycombing in concrete, characterized by rough textures and voids, is indicative of air voids present in the structure of the concrete. This diminishes the material’s strength and can expose reinforcements, compromising structural integrity.

Hard troweling of air-entrained concrete is discouraged as it leads to surface densification and strength loss subsequently reducing air content. Reduced surface air content in concrete due to hard troweling increases susceptibility to freeze-thaw damage and deicer-scaling, impairing durability.

Interference from Fly Ash

Fly ash, a by-product of coal combustion in power plants is often used in concrete for its beneficial properties. However, unburned carbon in the form of fly ash can adsorb air-entraining admixtures, diminishing their ability to stabilize air bubbles and therefore negatively impacting the process of air entrainment. When fly ash contains a significant amount of fine carbonaceous particles, it heightens interference with air entrainment.

Thankfully, post-treatment methods like ozonation, thermal treatment, and physical cleaning can enhance the quality of fly ash for use in concrete mixtures and reduce the adverse effects of air entrainment.

Future Developments in Air Entrainment

Looking ahead, several factors influence what constitutes air entrainment in concrete, and its evolution remains an ongoing process. Innovations such as MasterSphere, based on specially developed super-absorbent polymers, play a significant role. These innovations offer the potential for more reliable air-entrained concrete. As a result, they enhance the freeze-thaw durability of concrete.

The technology developed by MasterSphere ensures consistency in air entrainment, unaffected by variables such as raw materials, production parameters, or environmental factors.

Super-absorbent Polymers

Super-absorbent polymers (SAPs) are considered a pioneering replacement for traditional air-entraining agents, with the potential to enhance freeze-thaw resistance and durability in concrete. Some key features of SAPs include:

Capable of absorbing water up to 500 to 1500 times their weight
Retaining water within their polymer chains
Revolutionizing the way we think about air entrainment in concrete

Originally developed in the 1980s for use in the hygiene industry, SAPs have a wide range of applications across various industries, including medical, food packaging, and sanitary products.

Summary

So what is air entrainment in concrete? We’ve learned that air entrainment in concrete is more than just an intriguing scientific concept; it’s a critical factor in enhancing the durability, workability, and freeze-thaw resistance of concrete. Over the years, advancements in air entrainment, such as the use of air-entraining admixtures, have led to significant improvements in concrete performance. While challenges exist, solutions like proper placement techniques and mixture designs, as well as innovations like superabsorbent polymers, are paving the way for more reliable and durable air-entrained concrete. The future of air entrainment is indeed promising.

Certified MTP has the largest selection of concrete testing supplies, showcasing industry-leading brands for Concrete Compression Testing Machines, Concrete Air Entrainment Meters, slump testers, Concrete Test Cylinder Molds, and fresh concrete testing equipment.

Frequently Asked Questions

What is air entrainment in concrete?

Air entrainment in concrete serves to increase its freeze-thaw resistance by creating tiny chambers for water to expand into when it freezes. This helps protect the concrete from damage caused by freezing and thawing.

When should you not use air-entrained concrete?

You should not use air-entrained concrete for floors that require a dense, smooth, hard-troweled surface, as it may lead to blistering or delamination.

What is the difference between air-entrained and non-air-entrained concrete?

Air-entrained concrete has a higher air content compared to non-air-entrained concrete, which helps reduce the probability of microcracks by acting as expansion chambers for frozen water. This is due to the well-distributed air voids within air-entrained concrete.

How do you reduce air entrainment in concrete?

To reduce air entrainment in concrete, consider increasing the amount of fly ash per unit of concrete and adding carbon black as a colorant, which can decrease the air content. Be mindful that additional AEA may be needed to attain specified air levels when adding carbon black.

What is air entrainment in concrete and how is it achieved?

Air entrainment in concrete is achieved by using air-entraining admixtures that create microscopic air bubbles in the concrete mix. This improves the freeze-thaw resistance and workability of the concrete.